A tundish is a large intermediate holding vessel for molten metal used in continuous casting processes, such as the continuous casting of steel. The tundish is, in effect, an intermediate process reservoir which receives a large quantity of molten metal from conveying ladles exiting a furnace in which actual smelting of the ores or refining of molten metal occurs, and which then transfers the molten metal to a casting system. A system of inlet and outlet nozzles controls the flow of molten metal into and out of the tundish.
The tundish itself is generally a steel vessel which can be lined with several layers of various refractory compositions. A permanent lining, generally of refractory castable or brick, serves as an inner lining to protect the vessel. The permanent lining, in turn, is coated with a disposable lining, generally of a refractory composition which has been applied to the permanent lining by gunning, spraying, trowelling, or dry vibration. Alternatively, the disposable lining can be constructed of refractory boards. The disposable lining is in direct contact with the molten metal in the tundish and protects the permanent lining from exposure to or degradation by the molten metal.
In general, there are two categories of compositions used as disposable tundish refractory material. The first category includes those refractories which require the addition of a liquid phase, most commonly water, in order to allow the composition to be applied as a tundish lining. Compositions that require a liquid phase are generally applied by trowelling, gunning or spraying. A second category of refractory material may be applied in a dry phase, often requiring vibration of a mold in order to compact the material upon the surface of the permanent liner.
A dry vibratable refractory material may be applied by use of a former, mold or mandrel which corresponds to the contour of the inner dimensions of the tundish, or in a second method, may be applied directly to the permanent lining. The method not requiring a former, namely, the so called "no-former" method, involves preheating the vessel from about 1200.degree. to about 2000.degree. F. Once preheated, the vessel is completely filled with the dry refractory formulation. The refractory is allowed to cure in the vessel for about 1-5 minutes. After this time has elapsed, uncured refractory is reclaimed for reuse and the cured layer forms a substantially uniform disposable liner. By varying the time allowed for curing, the thickness of the disposable lining can be controlled to the amount desired.
During the drying or preheating of sprayable, gunnable, or trowellable refractory liners, there is the possibility that the water within the material may expand too rapidly and cause explosive spalling which can crack the lining. Dry-type refractory materials are not susceptible to such explosive spalling. However, due to the nature of the binders utilized in dry-type refractories of the past, some are less suitable in applications where pre-heating of the tundish is required. Most dry-type refractories experience complete burn-out of low temperature binders at temperatures of 800.degree. to 1100.degree. F. At these temperatures, the high temperature binders have not yet reached their effective binding capabilities. Thus, failures of some disposable liners of the dry-type have occurred when they have been pre-heated prior to casting.
A current problem with refractory materials is that they often contain alkali oxides such as, for example, Na.sub.2 O and K.sub.2 O. It is understood that, at casting temperatures, the alkali oxides present migrate to the disposable/permanent refractory interface. These alkali oxides adsorb and/or absorb onto or into the permanent refractory lining. With each successive application of conventional disposable linings, nephelite forms at the interface. Therefore, damage to the permanent lining can occur during removal of the disposable liner due to the fusion of the linings because of the nephelite formation. Sticking of the disposable lining to the permanent refractory can cause some of the permanent refractory to be lost from the metal vessel when the skull is removed. Furthermore, in those instances where the disposable lining does not stick to the permanent refractory, contamination of the permanent liner by the alkali oxides produces an increased disparity in the coefficient of thermal expansion. This difference in thermal expansion can also result in damage to the permanent lining.
What is needed, therefore, is a substantially alkali oxide free dry vibratable refractory composition which can be used to provide a disposable lining in casting vessels. This composition should also be formulated for use in processes requiring pre-heating of the tundishes such that the material does not exhibit significant spalling, cracking or other temperature-related lining failures.